People frequently carry substantial loads on their persons. Hikers, backpackers, skiers, and military personnel often use backpacks to carry loads. For backpacks lacking waist belts, the load is borne largely on the user's shoulders and back. For backpacks having waist belts, the load is typically shared between the user's shoulders and hips. Backpacks with frames, be they internal or external, are generally better than frameless packs at supporting and stabilizing large loads. Some backpacks utilize quasi-frames comprising wands inserted into pack receptacles, for supporting and stabilizing medium size loads without incurring the increased weight or range of motion limitations typical of more extensive frames.
A person who engages in a variety of outdoor recreational activities can require a relatively large collection of backpacks in order to have a backpack optimized for each activity. A lightweight pack having no waist belt or frame may be optimal for short day hikes or skiing inbounds at ski areas. A larger pack having a waist belt but no frame, or only carbon fiber wands for load stabilization, may be better for larger day hikes, backcountry skiing, and single overnight trips. For multi-day backpacking, a larger backpack having a substantial frame and waist belt may be ideal. Where a backpacker anticipates being out for a week or more, a super-large pack with a heavy duty belt and frame may be called for.
Military and law enforcement personnel frequently wear body armor such as ballistic vests, bulletproof vests, and flak jackets. The user's shoulders and back typically bear most of the weight of body armor. Military personnel frequently wear heavily loaded backpacks, and thus benefit from backpacks equipped with internal frames, external frames, quasi-frames, or other structural members that are rigid or semi-rigid and that assist in transferring some of the backpack load to a waist belt. However, the backpack frames are generally not adapted for use with body armor, with which a different frame or other support member, or no frame or support member, is employed. Moreover, backpack frames frequently restrict movement of the user because the frames are coupled to a waist belt and thus have little movement independent of the waist belt. For these and other reasons, military personnel sometimes carry very heavy backpacks in the absence of frames or other structural members that assist in transferring load to the users' hips, and back injuries from bearing heavy loads are consequently relatively common. Health care costs for the United States military related to such back injuries are large.
Military personnel sometimes need to wear body armor and carry a backpack simultaneously. With the load of body armor being carried primarily by the user's shoulders and thus transferred to the user's back, combining the backpack load with the weight of a ballistic garment can be most uncomfortable for the user. Back injuries can also be induced by this arrangement.